twenty-first appendage 

This is a sly reference to and synonym for the penis or cock. For males adding the penis to the 10 fingers and 10 toes makes 21 appendages. Use this if you want to sound intelligent or don't want stupid or younger people to know what you are talking about.

I'm in the process of writing an epic poem about the love between me and my twenty-first appendage.

twenty-first appendage by Kyle Bravlavski AKA Jewboy December 23, 2010

Twenty-First Century Gatsby 

Someone wealthy who throws parties like Gatsby today but with all the technology and fun that was around in the 20s and with the new culture.

Yeah he's the Twenty-First Century Gatsby.

Twenty-First Century Gatsby by André10000 October 11, 2011

tenty first 

Theres 21st and all but there was never an 11st so here it is.

welcome to the tenty first day of this month.

tenty first by Callsign X-Ghost February 12, 2021

twentyfirst base 

When you browse and masturbate to every base definition on urban dictionary

You: oh man I browsed every base definition on urban dictionary last night, I got all the way to twentyfirst base

Any rational human being: wow that's pretty sad bro

twentyfirst base by Thepeepeeman October 17, 2019